1. Field of the Invention
The present invention relates to an optical disc and in particular, to an optical disc having a land portion and groove portion, on both of which recording is carried out.
2. Description of the Prior Art
In the field of information recording, various groups are studying on the optical information recording method. This optical information recording method have a variety of merits, enabling to record and reproduce in a non-contact state and to obtain a high recording density than in a magnetic recording method by ten times or more, coping with various memory forms. The optical information recording method has been widely spread in practice for industrial use and civil use as a method enabling to realize a low-cost large-capacity file.
As a recording medium of reproduction-dedicated type, there are widely spread a digital audio disc on which a music information is recorded and an optical video disc on which a video information is recorded. Moreover, as a recording medium of writable type, there are spread a magneto-optical disc and an optical disc of phase change type.
There have been suggested various methods to increase the density of these optical discs. As one of these methods, there is an approach to record a data bn each of a land portion and a groove portion of a disc so as to realize a high density.
Normally, on an optical disc substrate is formed a continuous groove in a spiral form or coaxial form and a land formed between grooves. Conventionally, a data signal is recorded on either of the land portion or the groove portion as a recording track of the optical disc. This is because if recording is carried out on both of the land portion and the groove portion, a cross talk is caused, i.e., a signal is read from an adjacent track when reproducing an information.
However, from the viewpoint of recording density increase, it is by far more advantageous to utilize both of the land portion and the groove portion for recording a data signal. Consequently, Japanese Patent Application 5-282705 discloses an optical disc in which both of a land and a groove are used as tracks for recording by setting a groove depth Gd in the vicinity of xcex/6n (wherein xcex represents a laser wavelength used for data reproduction and n represents a refraction index of the substrate). Hereinafter, this method will be referred to as a land/groove method. In an optical disc having a groove depth Gd set to satisfy the aforementioned condition, when reproducing the groove, for example, even if a laser beam is radiated to an adjacent land, cross talk from the land can be suppressed to a extremely small value. The same applies to a case when the relationship between the land and the groove is reversed.
In the optical disc of the aforementioned land/groove method, a groove and land are formed on a substrate so as to have an almost identical width and a recording layer is formed on the groove and land thus formed.
However, this optical disc of the land/groove method has a problem that optical characteristics and other conditions differ between the land and the groove, disabling to carry out a preferable land/groove recording.
It is therefore an object of the present invention to provide an optical disc of the land/groove method in which optical characteristics and other conditions are equivalent in the land and the groove, which can be preferably used to carry out a preferable land/groove recording.
In order to achieve the aforementioned object, the inventor of the present invention studied on an optical disc appropriate for land/groove recording and found the cause of the aforementioned problem in the conventional land/groove type optical disc, i.e., different optical characteristics between the land and the groove formed on the recording layer. By removing the cause, the inventor has reached the present invention.
The land/groove type optical disc is prepared by forming a land and a groove so as to have an identical width on a transparent substrate made from polycarbonate or the like on which a recording layer is formed. A signal recording and reproducing is carried out by applying a laser beam from the surface of the disc not having the recording layer, and the laser beam is scattered and diffracted depending on the configurations of the land and the groove formed on the recording layer.
As shown in FIG. 1, in a conventional land/groove type optical disc, the recording layer formed on the slanting portion of the groove has a slightly thinner thickness and the groove formed on the substrate is half-filled with the recording layer, resulting that the groove formed on the recording layer has a smaller width than that of the land. Thus, even if the land and the groove of an identical width are formed on the substrate, the groove and the land formed on the recording layer have different width values and different optical characteristics.
In order to obtain identical optical characteristics between the land and the groove in the land/groove type optical disc, it is necessary that the land and the groove formed on the recording layer have approximately identical width.
Consequently, the present invention provides an optical disc having a substrate on which a groove and a land are formed so that a data signal is recorded on both of the land and the groove portions, the optical disc being characterized in that the groove formed on the substrate has a greater width than the land formed on the substrate and at least a recording layer is formed on the substrate where a data signal is recorded.
Furthermore, the inventor of the present invention used a typical schematic figure showing correspondence between the configurations of the substrate and the reflection film surface, so as to create an approximate expression which determines the optimal substrate configuration, and checked various optical discs to verify this approximate expression.
Firstly, as the typical schematic figure, the inventor used an optical disc as shown in FIG. 1 having a substrate on which a recording layer is formed.
Here, Wg represents an average width of a groove formed on the substrate; W1, an average width of a land formed on the substrate; Tg, a film thickness of a recording layer formed on the slanting portion of the groove formed on the substrate; To, a film thickness of the recording layer formed on the flat portion of the groove formed on the substrate; xcex8, an angle determined by the slanting portion and the flat portion of the groove formed on the substrate; and Tp, a sum of the average width Wg of the groove and average width W1 of the land formed on the substrate. As shown in FIG. 1, a groove formed on the recording layer after film formation has an average width smaller by 2xcex94 than the average value Wg of the groove formed on the substrate and as a result, a land formed on the recording layer after film formation has an average width greater by this value.
In the land/groove method, it is necessary that the groove formed on the recording layer after film formation have almost identical value as the average width of the land. Consequently, the following relationship should be satisfied.
Wgxe2x88x922xcex94=W1+2xcex94xe2x80x83xe2x80x83(2) 
If the track pitch is assumed to be Tp, then Tp=Wg+W1. This can be introduced into the Expression (2) to obtain the following expression.
Wg=0.5Tp+2xcex94xe2x80x83xe2x80x83(3) 
If the xcex8 is the angle determined by the slanting portion and the flat portion of the groove formed on the substrate, the xcex94 can be expressed as follows.
xcex94=(Tgxe2x88x92To)/tan xcex8xe2x80x83xe2x80x83(4) 
Thus, if the xcex8 and the Tg are known, the Wg value as the design target can be determined.
The value of theta can be defined by the following expression, assuming Ge as the width of the groove formed on the slanting portion and Gd as the depth of the groove formed on the substrate.
xcex8=tanxe2x88x921(Gd/Ge)xe2x80x83xe2x80x83(5) 
The values of Ge and Gd can be easily obtained from an AFM photograph, for example. Moreover, the Tg can be measured by taking a tomograph. As has thus far been described, the expression (3) was obtained for designing an optimal substrate for the land/groove method.
In the recording layer of a recording/reproduction optical disc, a dielectric film is formed so as to have a greatest thickness. The dielectric film is generally formed by way of the RF sputtering. In this process, for example, molecules contributing to form a film at a certain position are coming from various directions around that position. FIG. 2 shows a film formation model by the RF sputtering on a flat surface. At a certain position on the flat surface, particles coming from a hemispherical area are accumulated to form a film of thickness To.
On the other hand, FIG. 3 shows a film formation model by way of the RF sputtering on a slanting surface. As shown in FIG. 3, a volume corresponding to the leftward angle xcex8 does not contribute to film formation at that position. On the contrary, there are particles which come from the lower right in the figure. However, the particle amount coming from the lower right is smaller than the particles obstructed. As a result, the thickness Te of the recording layer in the perpendicular direction with respect to the groove slanting portion of the substrate can be interpreted to be thinner than the To. If the particle amount coming from the lower right is taken into consideration and a constant xcex1 is introduced, the Te can be expressed approximately by the following expression. Note that the unit of xcex8 in the expression is radian.
Te=(Pxe2x88x92xcex1xcex8)To/xcfx80xe2x80x83xe2x80x83(6) 
The xcex1 is a constant in a range of 0xe2x89xa6xcex1xe2x89xa61 depending on a film formation apparatus. This coefficient can be determined by repeatedly taking tomographs of the film formation substrate. This value was found to be 0.66 in the film formation apparatus actually used in the experiment.
Furthermore, from FIG. 1, the following expression can be obtained.
Tg=Te/cos xcex8xe2x80x83xe2x80x83(7) 
As has been described above, by determining the coefficient xcex1 in advance by the tomographs, the Tg can be calculated approximately from the Expressions (6) and (7).
Moreover, in general, the optical disc specification allows a fluctuation of about xc2x112% from a reference value for the reflection ratio fluctuation on the disc surface. In the land/groove method, if one of the land and the groove formed on the recording layer has a greater width, then the other has a smaller width, and the reflection ratio is changed. For this, it is considered that, corresponding to the allowance of the reflection ratio over the optical disc surface, there is an allowance for the width fluctuation of the groove and land. This allowance should be determined.
For this, a numerical experiment was carried out for the relationships between the fluctuation of the width of a rectangular groove formed on the recording layer and the reflection ratios of the land and the groove. FIG. 4 shows the results of this experiment. For the numerical experiment, Fourier analysis was used.
Here, the vertical axis represents the reflection ratio normalized by assuming 1 when the land and the groove formed on the recording layer have an identical width. Moreover, the horizontal axis represents the groove width fluctuation amount with respect to the track pitch normalized by assuming 0 when the aforementioned land and groove have an identical width.
As shown in FIG. 4, when the width of the groove formed on the substrate fluctuates, the reflection ratios of the groove and land formed on the recording layer also fluctuate proportionally. As a result of checking three patterns: Tp 1.4 xcexcm and xcex680 nm; Tp 1.6 xcexcm and xcex680 nm; and Tp 1.6 xcexcm and xcex780 nm (A/W=(1.1) in all the three cases), it can be said that the tendencies are matched with one another.
Furthermore, when the reflection ratio fluctuation allowance is limited to xc2x110%, as shown in FIG. 4, the groove width fluctuation amount can be allowed by about xc2x14% with respect to the track pitch. Consequently, the average width Wg of the groove formed on the substrate is preferably in the range defined by the following Expression (1).
0.46TP+2xcex94xe2x89xa6Wgxe2x89xa60.54TP+2xcex94xe2x80x83xe2x80x83(1) 
wherein xcex94=(Tgxe2x88x92To)/tan xcex8
It should be noted that the verification of the aforementioned Expression (3) will be described later.
In the optical disc having the aforementioned configuration according to the present invention, as the groove formed on the substrate is designed so as to have a greater width than the land formed on the substrate, it is possible that when a portion of the groove formed on the substrate is filled with a film formation, the groove and the land formed on the recording layer have an almost identical width, which enables to obtain equivalent optical characteristics for the land and the groove, enabling to be employed in the land/groove method.
Furthermore, by making the average width Wg of the groove formed on the substrate greater than the width of the land formed on the substrate so as to satisfy the aforementioned Expression (1), it is possible to maintain the reflection ratio fluctuation within the allowable range and make identical the width of the groove and the land formed on the recording layer, enabling to effectively design a substrate exhibiting identical optical characteristics for the land and the groove which can be preferably used in the land/groove method.